1. Field of the Invention
The present invention is a continuation in part of my U.S. Pat. Nos. 5,146,755 and 5,133,190 which disclosed methods for clean-up of a boiler flue gas stream by cooling and condensing its acidic water vapor, and for separation and liquefaction of sulfur dioxide and carbon dioxide gases. More particularly, it relates to an improved method for cooling and condensing the flue gases, while preheating the boiler combustion air by employing a heat pipe heat exchanger, and a method for liquefying, and removing the pollutant gas products at lower pressure and temperature ranges, by employing a gas compressor-expander in an open heat cycle system.
2. Description of the Prior Art
In my U.S. Pat. No. 5,146,755, I disclosed a method for flue gas cleaning system employing a power co-generation system, to conserve energy, and provide-cooling for the hot flue gas stream and condensing its acidic water vapor. In my U.S. Pat. No. 5,133,190 I disclosed another system for separating the cooled flue gas stream into a lighter gas fraction enriched with nitrogen, and a heavier gas fraction enriched with carbon dioxide and sulfur dioxide gas components. The lighter gas fraction is diffused in a cooling tower system to dissolve any trace nitrogen oxides and sulfur dioxide emitted in the lighter gas fraction, and to uplift and disburse the gases into the atmosphere. The sulfur dioxide, and as desired part of the carbon dioxide are liquified by compression and cooling processes. The relatively high pressure of the uncondensed gases is reduced to allow venting into the atmosphere.
One embodiment of the present invention provides an improved method employing a heat pipe heat exchanger to recover the low temperature heat energy rejected in the flue gas stream to preheat the combustion air stream, while cooling and condensing its acidic water vapor. The system which herein is referred to the condensing boiler systems will achieve a substantially higher thermal efficiency, and will recover wasted energy recovered to preheat the combustion air, and will result in reducing the heat rate of the steam power plant. This is contrast to a separate low temperature co-generation electric power system employing an ammonia driven power cycle which operates at much lower thermal cycle efficiency, when compared with the utility high pressure steam power cycle.
In the past, gas fired high efficiency condensing furnaces are used for residential and commercial space heating systems. In a residential or commercial condensing furnace, the flue gas stream is cooled, and its water vapor is condensed by exchanging its sensible and latent heat with the recirculating room air stream. The present invention provides a high efficiency condensing boiler system employing a reversible heat pipe heat exchanger. In the heat pipe heat exchanger structure the flue gas stream is cooled and its water vapor is condensed by exchanging its sensible and latent heat with a combustion air stream flowing in a reverse direction.
Another embodiment of the present invention overcomes the high power requirements for the liquefaction of the sulfur dioxide, and carbon dioxide, and provides novel means for condensing the sulfur dioxide, and the carbon dioxide while recovering wasted energy, thereby providing an energy-efficient system by employing a gas compressor-expander in an open heat cycle system, First; to condense its acidic water vapor containing dissolved nitrogen oxides and other volatile organic vapors, and Second; to liquefy most of the sulfur dioxide and carbon dioxide contained therein at much lower temperatures utilizing the cryogenic effect from the gas expansion. This is contrast to the gas liquefaction by compression and cooling using water and auxiliary refrigeration system, for which higher pressures are needed to achieve the thermodynamic equilibrium temperatures, not making advantage of the wasted heat energy released with the uncondensed clean gas stream.
An open heat cycle system is employed for liquefaction process, where a gas compressor-expander acts upon the heavier gas fraction flowing from the gas separation step, to change its pressure, and temperature to reach a thermodynamic equilibrium point corresponding to condensing a gas component. In the course of this process, and in the presence of excess oxygen; the trace nitrogen oxide content in the flue gas is subjected to a dramatic accelerated oxidation, and reacts immediately in the water vapor condensate to form weak nitric acid.
The type of the gas compressor-expander will depend upon the desired operating pressures and temperatures. The gas compressor-expander unit may be of centrifugal or axial flow, single or multi-stage, and its drive may be electric motor, or steam turbine. The cryogenic effect produced by the gas expander is regulated by controlling the pressure ratio across the expander. Raising the gas compressor discharge pressure increases the pressure ratio, and provides more refrigeration effect needed for the liquefaction of the carbon dioxide. The working pressure and temperature range must be controlled to prevent forming solids or icing of carbon dioxide at or near the gas subliming equilibrium point.
Pressure ratio of up to 3:1 with the range of 2:1 to 2.5:1 will be preferred for liquefaction of the sulfur dioxide, and pressure ratio of up to 20:1 with the range of 10:1 to 15:1 will be preferred for liquefaction of the carbon dioxide.
In the past, open air heat cycles in different arrangements have been used for many years for environmental control and liquefaction of gases in many industrial applications including oil refinery, nitric acid manufacturing, and air liquefaction processes. U.S. Pat. No. 4,923,492 disclosed a system for removing hydrocarbons or chemical vapors; U.S. Pat. No. 4,696,689 disclosed a system for separating product gas from raw gas; however, in so far as is known, the invention described herein is first to employ a compressor-expander open heat cycle system to clean flue gases, by cooling and condensing its nitrogen oxides, sulfur dioxide, carbon dioxide and other organic gas compounds emitted from the fossil fuel fired boilers power plants.